The present invention relates to protective relay systems for protecting a three-phase electrical power transmission segment of a power system network against faults, in general, and more particularly to a protective relay system operative to perform selective-pole trip determination utilizing positive, negative, and zero sequence components of the phase currents monitored at each end of the segment.
In an electrical power system network, electrical energy is generally transmitted from the one point to another over one or more three-phase transmission segments. To protect the power system, some electrical utilities are presently using current differential relay systems, like the LCB manufactured by Westinghouse Electric Corporation, for example, to sense internal faults within the three-phase segments and isolate the faulted segments from the network. In these LCB type systems, protective relays and corresponding power circuit breakers are coupled at each local and remote end of a transmission segment. The local and remote protective relays communicate with each other over one or more communication channels to detect a fault internal to the segment which they are protecting and to operate the corresponding circuit breakers to interrupt current through the protected segment in response to a detected internal fault.
A typical current differential relay system is disclosed in the U.S. Pat. No. 4,275,429 issued June 23, 1981 to Larry L. Church et al. and entitled "Protective Relay Apparatus" which is incorporated by reference herein for the purpose of providing a more detailed description of such a relaying system. Briefly, the aforementioned patent teaches that each local and remote protective relay of a protected line segment monitors its corresponding phase currents and generates signals representative thereof. Each set of phase current signals are converted into local and remote composite sequence signals which are transmitted correspondingly between the local and remote protective relays through a communication channel. Each protective relay compares the local and remote composite sequence signals to detect an internal fault in the protected segment.
One drawback of the LCB or an equivalent relay system is that it cannot segregate an internal fault to a particular phase of the protected segment, rather it detects only an internal fault and causes a three-phase fault trip. It is preferred to have independent-pole operation of the power circuit breakers in which case the faulted phase or phases of a segment would have to be individually and separately detected, the operation of which is commonly referred to as selective-pole trip determination. In such a protective relay system, the circuit breaker at each end of the protected segment can be operated to independently trip and clear only the faulted phase or phases of the segment leaving the other unfaulted phase(s) in service.
Typical of the current only monitoring relay systems which are capable of providing selective-pole trip determination is the segregated phase comparison units (SPCU) manufactured by Westinghouse Electric Corporation and described in the Westinghouse Paper "A New Relaying System to Protect Series Compensated Lines" authored by W. L. Hinman et al. and presented to Georgia Institute of Technology at the Protective Relaying Conference, May 3-4, 1973. Briefly, Westinghouse's SPCU system compares the monitored phase currents of one end of the protected segment with respectively corresponding monitored phase currents of the other end of the segment at each protective relay to detect an internal fault and isolate the faulted phase. However, to accomplish the three-phase current comparisons, the SPCU system requires three separate communication channels or some sophisticated multiplexing arrangement for a single communication channel, both configurations posing other undesirable conditions.
Another drawback resulting from current only monitoring protective relaying systems, such as the SPCU, for example is isolating the faulted phase under the conditions of heavy load current or high fault resistance or a combination thereof. If the load current is too large, the protective relay may be fooled into determining falsely that an actual internal fault is an external fault and defor tripping. A similar condition may occur with high impedance faults in which case the faulted current may be substantially smaller than the load current resulting in a sustaining current outfeed from the protected segment. A combination of both high current and high fault resistance merely compounds the problem. In fact, such a combination may, in some cases, shift the phases of the currents in such a fashion as to make a phase-to-ground fault appear as a phase-to-phase-to-ground fault.
Accordingly, applicant's invention is intended to improve upon the present current only monitoring protective relaying systems which offer selective-pole trip determination operation, especially with regard to the aforementioned drawbacks.